A conventional d.c. motor for use in an engine starter is constructed as illustrated in FIG. 1, from which it is seen that the conventional d.c. motor comprises a rotary shaft 3 on which an armature core 2 is firmly secured by means of splines 1 formed on the circumferential surface of the rotary shaft 3. A commutator 4 is mounted on one end portion of the rotary shaft 3.
The commutator 4 comprises a plurality of commutator segments 5 made of copper mounted at equal circumferential intervals on a resin portion 6 made of an electrically insulating material which is secured on the rotary shaft 3 through a bushing 8. The commutator 4 comprises a cylindrical sliding contact portion 4a on which brushes (not shown) of the motor are brought into a sliding contact relationship and a large-diameter connecting portion 4b provided at a first end of the sliding contact portion 4a. Each of the commutator segments 5 extends in a substantially axial direction over both the sliding contact portion 4a and the connection portion 4b. Radially outer ends of the connection portions 4b of the segments 5 are electrically connected to an end of an armature coil 7 wound on the armature core 2.
The armature coils of many of such d.c. motors are made of an aluminum conductor due to its lightweight, and the mechanical and electrical connection between thee aluminum coil conductor and the copper connection portion 4b of the commutator segment 5 has been achieved by the ultrasonic welding of these two different materials.
However, it is difficult to obtain sufficient mechanical strength between the aluminum conductor and the copper segment through ultrasonic welding. Also, the weld is inferior in terms of corrosion resistance.
While it has been proposed to interpose a layer such as of nickel or tin between the aluminum. conductor and the copper segment, the mechanical strength obtained has not been satisfactory.